We discourage thinking too much about "Plan B," because should you really plan on your "Plan A" failing?
But Plan B is usually what happens. Plan A is based on assumptions, numbers, conditions that are incorrect. So you have to be flexible to morph into Plan B. And not feel badly about it.
But a backup plan would also be based on incorrect assumptions, so better to form Plan B as you move along. Start out with Plan A, then shift as needed.
You might end up with a completely different business, but that puts you in good company. Some of the greatest businesses of our age also did. Completely shifting business models after startup is actually actually a business model. It is, in fact, typical.
(See Getting to Plan B: Breaking Through to a Better Business Model).